Possible origins of the Antlia 2 galaxy? At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on their proper motions as measured by the Gaia mission:

The discovery article includes a discussion of possible origins, but ultimately states caveats to all proposed scenarios. How could such an extreme galaxy come to exist?
 A: I don't think it's possible to give a single completely confident answer to this question yet.  However, this paper presents some interesting evidence for one of the leading theories.  The authors recalculate the proper motions of each known star belonging to Antlia 2 and extrapolate the trajectories backward in time to estimate the pericenter of the galaxy's orbit, i.e. the point at which it came closest to its center of gravity with the Milky Way.  They found that an orbit with a low pericenter, less than 10 kpc, is plausible.
But the Milky Way is about 15 kpc in radius!  So if the pericenter is really that low, it would mean that Antlia 2 collided directly with the Milky Way.  That's not as dramatic as it sounds.  Since galaxies are very sparsely populated with stars, it's likely that the two galaxies would have simply passed through one another with very few stellar collisions.  However, gravitational interactions would have pushed Antlia 2's stars in random directions, causing them to spread out over a large volume.  That fits the observations neatly.  There are some nice visualizations in an article here.
The collision theory would also solve another mystery, the otherwise difficult to explain ripples in the outer disc of the Milky Way.  Since the stellar trajectories are not yet precisely determined, though, we cannot draw a firm conclusion without more data.
